1. Field of the Invention
This invention pertains to the elimination of polarization fading in Unbalanced Fiber Optics Pulsed Measuring Interferometers. It is believed to be classified in Class 356/Subclass 345.
2. Definitions of Terms Used Herein
An Optical Interferometer is an interferometer that uses interference of light signals to produce other light signals carrying information.
A Fiber Optical Interferometer is an interferometer that carries light signals in an optical fiber.
A Measuring Interferometer is an interferometer that produces signals that are measures of pressure, temperature, sound waves or other physical parameters.
A Pulsed Interferometer is an interferometer driven by a pulsed signal.
A Multiplexed Interferometer is a number of interferometers which are interconnected and use a common optical source and/or optical detector.
Time Division Multiplexed Interferometer is an interferometer whose signals are time-multiplexed. That is, an interferometer whose signals are displaced in time. All of the interferometers mentioned herein are Time Division Multiplexed Interferometers.
An Unbalanced Interferometer is an interferometer wherein a light beam takes longer to travel the length of one interferometer branch than it does to travel the length of another interferometer branch. Usually the unbalance occurs because of the different lengths of the signal paths of the two branches.
A Compensating Interferometer is a two-branched Unbalanced Interferometer that has in one of its branches a predetermined time delay sufficient to produce two predetermined time-separated pulses.
A Polarization Controller is an apparatus that controls the polarization of the light signal travelling therethrough. In the state of the art it may, for example, be comprised of an electrode array positioned adjacent an optical fiber carrying a light signal.
A Phase Controller is an apparatus that controls the phase of the optical signal therethrough. Various apparatus may be used to modulate or vary the phase of the optical signal through predetermined phase angles.
A Pulse Sequence is a group of sequential pulses.
A Two-Pulse Sequence is a sequence of two pulses.
A Driving Group of pulses is a plurality of sequential spaced-apart Pulse Sequences that drive the input of a Measuring Interferometer. In the apparatus of the invention, a Driving Group from the Compensating Intereferometer has two two-pulse sequences.
A Ladder Network, for purposes of this specification, is a network of fiber optical elements. The sensing elements are serially positioned in the input fiber, and shunt optical fibers are positioned to shunt optical signals from the two ends of the string of sensing elements and from the junctions between the sensing elements to the output fiber.
A Measuring Pulse Group is a series of light pulses, containing sensed information, at the output fiber of a Measuring Interferometer.
Interference Visibility is the extent of the possible variation in output pulse amplitude caused by interference of two pulses in an interferometer.
Polarization Fading occurs when the polarizations of two concurrent pulses on the output fiber are orthogonal to each other, and the visibility is zero. When the two polarizations are identical, the two pulses interfere, and the visibility of the pulse produced from that interference is maximum. Between being identically and being orthogonally polarized, the pulse produced by the two interfering pulses has less than maximum visibility. The phenomenon of reduced visibility of a Measuring Interferometer, due to the Polarization Angle between concurrent pulses, is called Polarization Fading.